Boil spaghetti in salted water until al dente (8–10 minutes). Drain and set aside.
Brown the meat in a skillet, drain fat, then stir in marinara. Season with salt and pepper. Mix in spaghetti until well coated.
Preheat oven to 375°F (190°C). Slice bread lengthwise and hollow out the bottom half to form a boat.
Stuff the spaghetti mixture tightly into the bread. Top with mozzarella and Parmesan.
Mix melted butter, garlic, and parsley. Brush over the top bread half and edges of the stuffed loaf.
Place both halves on a baking sheet. Bake for 10–15 minutes until the cheese melts and bread turns golden.
Place the top half over the spaghetti loaf, press gently, slice thick, and serve warm.